1. Field
The present invention relates to an angular speed sensor correction technology and, more particularly, to an angular speed sensor correction device and an angular speed sensor correction method for correcting outputs from an angular speed sensor.
2. Description of the Related Art
In general, on-vehicle navigation devices are configured to estimate an optimal position by synthesizing the position calculated from a self-contained navigation system and the position calculated from a Global Positioning System. In a self-contained navigation system, the current position is calculated by updating the previously measured position based on the speed pulses indicating the speed of the vehicle and the angular swing speed of the vehicle measured by the angular speed sensor. The navigation devices of a scheme as described above is capable of deriving the position of the driver's vehicle even in tunnels, underground parking lots, or areas between tall buildings, in which it is difficult to receive radio waves from a GPS satellite, by using the self-contained navigation system. The angular speed co of a circling vehicle can be derived according to the following expression.ω=(Vout−Voffset)/S  (1)where Vout denotes an output voltage of the angular speed sensor, Voffset denotes an offset value of the angular speed sensor, and S (mV/deg/sec) denotes a sensitivity coefficient of the angular speed sensor.
In order to determine the angular speed accurately, it is necessary to determine the offset value and the sensitivity coefficient of the angular speed sensor. The sensitivity coefficient of the angular speed sensor generally varies depending on the difference between individual angular speed sensors and the angle of mounting the angular speed sensor to the vehicle. The offset value may change with temperature. More specifically, the offset value is affected by temperature change due to heat dissipated from a substrate, etc. used in the on-vehicle navigation device and heat dissipated by the vehicle engine etc. when the on-vehicle navigation device is mounted to the dashboard of the vehicle. In the related art, the offset value of the angular speed sensor is corrected by the voltage output from the angular speed sensor when the vehicle comes to a stop or is traveling in a straight line, when the angular speed becomes “0”. However, in cases where the frequency that the vehicle comes to a stop is small, such as when the vehicle is traveling on a high way or traveling for a long period of time in an area with little traffic, it is difficult to correct the offset value of the angular speed sensor periodically. As a result, the prevision of the offset value is likely to become poor. Offset value correction performed while the vehicle is traveling in a straight line to cause the output voltage from the angular speed sensor to become “0” accurately is also affected by road conditions or conditions of driving by the driver. It is therefore difficult to perform accurate offset value correction periodically. The sensitivity coefficient of the angular speed sensor is derived from the amount of orientation change in a unit period of time and the output voltage from the angular speed sensor. For this reason, the sensitivity coefficient of the angular speed sensor is affected by errors in the offset value, as is evident from from expression (1).
There is proposed a technology to correct the offset value and the sensitivity coefficient of an angular speed sensor even while the vehicle is not traveling in a straight line. According to this technology, the offset value and the sensitivity coefficient of the angular speed sensor are corrected based on the average value of output voltages of the angular speed sensor over a predetermined period of time and the amount of orientation change of the vehicle during the period of time over which the average value is calculated. More specifically, the offset value Voffset of the angular speed sensor is calculated as follows.Voffset=1/n·ΣVout−1/Δt·Δθ/n·S  (2)where n denotes the number of samples of output voltages of the angular speed sensor, Δt (sec) denotes a sampling interval, and Δθ(deg) denotes the amount of orientation change. The amount of orientation change is determined based on a GPS-based orientation acquired from a GPS satellite or on map data. The sensitivity coefficient of the angular speed sensor is derived as follows while the amount of change of the corrected offset value is small, i.e., while the amount is stable.S=(1/n·ΣVout−Voffset)·n/Δθ·Δt  (3)where Voffset is assumed to be known and a constant value in a stable condition in which the amount of change of the corrected offset value is small (see, for example, patent document 1).
There is also proposed a technology to correct the offset value of an angular speed sensor even in a situation where radio waves from a GPS satellite cannot be received, such as when the vehicle is traveling indoors. According to this technology, the relationship between the temperature measured by a temperature sensor and the offset value is approximated by a function, and the gradient of the offset value in the neighborhood of the measured temperature is calculated accordingly. More specifically, the offset correction value ΔVoffset in the presence of temperature change is calculated as follows based on the gradient of the offset value.ΔVoffset=β·(Tnow−Told)  (4)where β(mV/° C.) denotes the gradient of offset, Tnow denotes the temperature currently measured, and Told denotes the temperature previously measured. By adding the offset correction value to the offset value determined when the temperature was measured previously, the current offset value is derived (see, for example, patent document 2).    [patent document 1] Japanese Patent Application Laid-open No. 2001-330454    [patent document 2] Japanese Patent Application Laid-open No. 2009-192495
If the frequency of correcting the offset value is increased in this situation, it may take time to track the true offset value unless the resolution of the temperature sensor is sufficient. For example, if the resolution of the temperature sensor is 0.1° C., the offset correction value cannot be detected unless the difference between Tnow and Told in expression (4) is 0.1° C. or greater. Therefore, it takes time before the offset value is corrected such as when the temperature inside the vehicle is increased only slightly. In order to increase the precision of deriving the angular speed in a situation where the ambient temperature changes mildly, it is required to derive the offset value of the angular speed sensor in a short period of time and with a high precision.